HHS says stockpiled H5N1 vaccine is losing potency

Editor's note: The day after publication of this story, a federal official told CIDRAP News the amount of H5N1 vaccine in the US stockpile that had begun to lose potency was less than 20% of the total, not a majority of the doses. See the Nov 17 follow-up story for more details.

Nov 16, 2006 (CIDRAP News) – Federal health officials say much of the H5N1 avian influenza vaccine bought for the US stockpile is losing potency, with the result that the number of people who could be vaccinated has dropped by about 1 million since July.

A pandemic planning report released by Health and Human Services (HHS) Secretary Mike Leavitt this week says the stockpile contains enough courses of H5N1 "clade 1" vaccine to vaccinate about 3 million people. The previous update, released in early July, said there was enough to vaccinate about 4 million. The stockpile is part of US preparation for a potential flu pandemic.

Bill Hall, an HHS spokesman in Washington, DC, said a loss of vaccine potency is the main reason for the decrease in the number of people who could be immunized. He said a small percentage of the stockpile has been used for research.

"All vaccines have shelf lives," Hall told CIDRAP News. "The early vaccine that was purchased, the first lots, have begun to lose their potency."

He said the potency has begun to decrease for "the majority" of doses in the stockpile, adding, "That doesn't mean it goes from 100 percent to zero percent" or that the doses would be unusable.

But if the vaccine needs to be used, the remaining full-potency vaccine will be used first, Hall said. "If we use that up, then we may potentially use the vaccine that has lower potency."

Hall said not very much is known about the shelf life of flu vaccines, "because the main experience is with seasonal vaccine. At the end of the season the vaccine that's not used is thrown away. We don't keep it around, so no one really knows how long it would stay good for." With many flu virus strains circulating at any given time, seasonal flu vaccines have to be adjusted every year to match the strains expected to be dominant.

A spokesman for Sanofi Pasteur, maker of the H5N1 vaccine in the US stockpile, could not be reached in time to comment on the reported loss of potency.

The vaccine, based on a clade 1 H5N1 virus collected in Vietnam in 2004, yielded modest results in a clinical study conducted last year. About half of volunteers who received two 90-microgram doses had an immune response that was considered protective. That's a much larger dose than is used in seasonal flu vaccine, which usually contains 15 micrograms for each targeted flu strain.

When HHS released its pandemic plan in November 2005, it listed a goal of stockpiling enough H5N1 vaccine to cover 20 million people.

Earlier this year, HHS announced plans to develop and stockpile a vaccine based on a clade 2 H5N1 virus that was isolated in Indonesia in 2005. Clade 2 viruses have circulated in Europe, Africa, and parts of Asia.

The latest pandemic plan update says, "HHS is moving forward with the development of H5N1 clade 2 vaccine candidates and the manufacturing of H5N1 clade 2 vaccine stockpiles. Depending on the availability of vaccine seed stocks and production yields, we expect to have another five million courses of H5N1 clade 1 and clade 2 vaccines stockpiled in 2007."

Two weeks ago, a group of experts assembled by the World Health Organization recommended that governments not rush to stockpile "pre-pandemic" flu vaccines, because too many major scientific questions about them remain. They said that vaccines that seem to work well against one H5N1 clade don't work well against others. Also, they said no one knows what level of measured immune response indicates an adequate level of protection.

The pandemic planning report released this week, called Pandemic Planning Update III, also discusses stockpiling of antiviral drugs. So far HHS has stored 16 million treatment courses of antivirals, en route to a goal of 26 million courses by the end of this year and 36 million by March 2007, the report says. The longer goal is to have 50 million courses by the end of 2008.

HHS is buying both oseltamivir (Tamiflu) and zanamivir (Relenza) for the stockpile, according to Hall. He was unsure exactly how much of each is on hand now, but said the goal is for the stockpile to be about 80% oseltamivir and 20% zanamivir.

In addition to buying antivirals directly, HHS is offering a 25% cost-share to help states buy their own supplies, up to a total of 31 million courses. The report says 33 states have ordered antivirals under that program, and only four states don't plan to take full advantage of the subsidy. Hall declined to name those states, saying it's up to them to release the information.

Leavitt's planning update says HHS also has been stockpiling personal protective equipment. The government now has 73.1 million N95 respirators, used to protect healthcare workers from airborne pathogens, with another 31.8 million on order. The stockpile also contains 37.4 million surgical masks, along with face shields, gloves, and gowns, the report says. The previous update, in July, listed 20.2 million respirators and 12.3 million masks.

See also:

HHS pandemic planning update
http://www.flu.gov/professional/pdf/panflureport3.pdf

Mar 30 CIDRAP News story "H5N1 vaccine trial shows limited benefit"

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